1. Field of the Invention
This invention relates to acoustic transducers for use in pulse-echo acoustic ranging systems, and more particularly to transducers of the flexural mode type.
2. Review of the Art
Commonly assigned U.S. Pat. No. 4,333,028 (Panton), issued Jun. 1, 1982, describes a flexural mode transducer suitable for use in pulse-echo acoustic ranging systems, and also discusses prior art flexural mode transducers such as those described in an article in Ultrasonics, November 1978, "An Ultrasonic Transducer for High Power Applications in Gases", which had characteristics such as extremely high Q which rendered them unsuitable for use in echo-ranging applications. The Panton transducer has been very successful in a wide range of applications, but problems have arisen in certain applications due to difficulties in finding materials to form the matching rings applied to the transducer plate which exhibit consistent acoustic properties and provide good performance over extended intervals in applications involving extreme temperatures (high or low) and/or aggressive atmospheres.
In order to meet these problems it has been proposed in U.S. Pat. No. 4,768,615 (Steinebrunner et al) to replace the matching rings used by Panton by a rigid, apertured masking plate in front of the flexural oscillator plate, defining annular rings which mask the radiation from adjacent antinodal zones of the oscillator plates , whilst air between the rings formed low-loss coupling means matching the remaining zone of the plate to the atmosphere. Whilst such a masking plate can readily be made resistant to extreme temperatures and aggressive atmospheres, the arrangement is inherently less efficient than those preferred embodiments of the Panton arrangement which seek to match the phases of radiation from adjacent antinodal zones, since the radiation from alternate antinodal zones is necessarily lost, and the coupling of the remaining zones by the air between the rings renders it less easy to obtain a system Q which is low enough to provide a rapid ring-down of the transducer following transmission of a ranging pulse. Furthermore, it is difficult to provide adequately against particulate material becoming trapped between the masking plate and the oscillating plate, with deleterious effects upon the performance of the transducer.